Recent concern for the welfare of the world population has included consideration of additional means for feeding the rapidly increasing number of people involved. The problem embraces providing both adequate per capita caloric intake and a balanced diet, with particular reference to the acknowledged lack of sufficient protein-affording foods in many parts of the world. One means for providing necessary protein supplies is through the growth of single-cell protein-affording microorgansims, such as yeasts, bacteria and algae, for use as either foods or food supplements.
Production of single-cell protein (SCP) materials in large quantity may be accomplished by fermentation processes employing, for example, carbohydrate, hydrocarbon or oxygenated hydrocarbon materials as substrate. Principal requirements are that the substrate material be inexpensive and readily consumed by the selected microorganism so that process costs are not excessive. Equally important is the acceptability and utility of the SCP material as a food or food component. These latter considerations include taste and odor factors relating to public acceptance as well as metabolic and toxicity factors relating to suitability of SCP material for inclusion in the human diet.
Both the technical and the patent literature describe fermentation processes for production of microorganisms which readily afford useful SCP materials. For example, yeasts have been grown on the carbohydrates contained in waste sulfite liquor, the normal alkane components of a gas oil hydrocarbon fuel, and on a mixture of oxygenated hydrocarbons. Production of bacteria has been similarly described. Fermentation to produce yeasts or bacteria comprises an oxidation process, evolving much heat and requiring both substantial oxygen transfer and good control of fermentation temperature. Preferred substrate materials will already contain as much combined oxygen as possible in order to minimize the heat release and the oxygen requirement. Production of foodgrade SCP material may also require an extraction step to limit the presence of undersirable, residual substrate material such as high-molecular-weight hydrocarbons or slowly fermented oxygenated hydrocarbon species.
A number of the fermentation processes planned or in use currently for production of SCP material are intended to provide primarily an animal feed supplement and hence to supply protein for human consumption only indirectly. However, some microorganisms, notably certain yeasts within the Saccharomycetoideae and Cryptococcoideae sub-families, have been approved by the Food and Drug Administration for direct use in foods intended for human consumption.
SCP materials may be utilized as whole cells or may be processed to recover protein or protein hydrolyzates for inclusion in various food products. The whole-cell material typically is deficient in desirable functional properties, being powdery small cells (1-10 micron dimensions) which do not form a cohesive structured mass. The protein content of cells may largely be recovered by rupture or destruction of the cell wall, as by application of shearing forces or autolytic treatment with enzymes. Such proteins are usually concentrated by adjusting the pH to the isoelectric point and separating as by filtration or centrifugation. The hydrolyzates comprise chemically degraded protein material rich in the amino acid moieties which normally combine together to form the protein macromolecules.
Desirable functional properties in SCP materials include lowdispersibility in water, good water retention, oil absorption and retention, heat coagulation and emulsion stabilization. Water-soluble protein materials, intended for replacement or supplementation of egg albumin, should exhibit good whippability providing a large and stable foamed mass.